1. Field of the Invention
This invention pertains to the field of subjective measurements and characterizations of an eye, and more particularly, to measurements and characterizations of an eye with wavefront analysis devices.
2. Description
FIG. 5 is a diagram of a human eye 500, illustrating the choroid 510, the fovea layer 520, the lens 530 and the cornea 540. The fovea layer 520 is the area of the retina that contains the densest concentration of photoreceptors.
The location of the photoreceptors in the human eye is difficult to determine accurately relative to other structures in the retina. The photoreceptors absorb visible light and they are transparent to infrared light, making an accurate measurement of their location difficult.
However, there are a number of contexts where an accurate determination of the location of the photoreceptors would prove very beneficial. In particular, as explained in more detail below, it would be beneficial to provide a system and method for accurately measuring the choroid to photoreceptor distance in the eye.
The retina requires a constant supply of blood for it to remain healthy, and in fact consumes the greatest amount of oxygen (per weight) of any tissue in the human body. Evidence from several sources show that the photoreceptor layer ranges from 0.1 mm to 0.4 mm from the choroidal blood supply in normal human eyes. The average distance between the choroid and the cones in the fovea is 0.2 mm. Although this distance has not been well studied before, it seems reasonable to suspect that in some individuals an abnormally large distance from the nourishing choroid to the photoreceptors may predispose them to visual defects resulting from sight degrading diseases such as glaucoma and diabetes. Early identification of abnormal photoreceptor to choroid spacing may result in improved patient health.
The retina is supplied with blood by two means. The inner two-thirds is nourished by branches from retinal vessels on the inner surface of the retina, while the outer one-third is nourished by the choroid. However, the fovea centralis is nourished solely by the choroid. Evidently, any overlying retinal vessels in the region of the fovea would block light from reaching the photoreceptors and would result in greatly reduced visual acuity. Since the choroid is the sole nourishment of the fovea, any abnormalities in the choroidal blood supply could result in reduced diffusion of oxygen and nutrients into the fovea and that might cause cell damage and reduced vision. While a large spacing would not normally result in visual disfunction, if the patient were to develop diabetes or glaucoma, the patient could experience an unusually rapid progress in decline of visual function. Such patients would need to be monitored more closely. Similarly, some populations, such as Native Americans, are normally more closely monitored for the onset of diabetes. It may be that data concerning choroid to photoreceptor spacing would indicate that only a small subset of those populations are actually in need of close monitoring and examination intervals could be increased for the remainder of those populations.
A second application of this measurement of the choroid to photoreceptor distance would be to improve the accuracy of objective refractive measurements that are used to determine a patient's eyeglass prescription. Many commercially available autorefractors and wavefront aberrometers reflect light off of the choroid (or the sclera) to provide a light source for such an automatic measurement. Technicians rather than highly trained doctors can perform autorefractions, and the autorefractions are faster to perform than subjective refractions.
Most people are familiar with the process of subjective refraction when an ophthalmologist flips lenses of different strengths in and out of the patient's field of view and asks the patient if a letter on the wall looks clearer or fuzzier. The repeatability of subjective refraction is generally considered to be about +/−0.25 Diopters. A few practitioners with better skill and more time to spend with the patient can achieve repeatabilities of +/−0.12 diopters.
Meanwhile, the repeatability of autorefraction measurements is better than +/−0.1 Diopters for almost all modem autorefractors, and that repeatability is much better than that of subjective refraction
However, according to several review articles, about 20% of patients will have differences between a subjective refraction and an autorefraction greater than 0.5 Diopters. Significantly, repeated autorefractions performed on a particular patient from the 20% group will consistently give the same disagreement with subjective refraction, so clearly there is some kind of structural difference in those patients eyes relative to the general population.
Eyeglasses prescribed according to subjective refraction meet with greater patient satisfaction than those that would be prescribed according to autorefractors. Those patients that had more than an 0.5 Diopter discrepancy will almost always be unhappy with eyeglasses prescribed according to the autorefraction, and happy with the eyeglasses prescribed according to subjective refraction.
Accordingly, it is standard practice in the evaluation of autorefractors to consider subjective refraction to be the “gold standard” since it correlates better with patient visual experience than any other measurement. Consequently it is standard practice for ophthalmologists to fine-tune the autorefraction values by performing a subjective refraction on the patient using a phoropter. The result is that autorefractors are only used for screening purposes, or for giving an optometrist or ophthalmologist a good starting point in doing a subjective refraction.
Objective refractors use infrared light to measure the eye because infrared light reflects much more strongly out of the eye than visible light does. However, there are fundamental physical reasons to expect problems with the approach of using an infrared light beam to autorefract a patient or subject.
First, when an objective refraction measurement is performed, the light is scattered back to a measurement device from a location in the eye that is not the same as the location of the photoreceptors in the eye. That means that the above-described autorefraction measurement is referenced to a location that does not correspond to where photons are being converted into neural impulses. It has been theorized that the infrared light scatters off the choroid in the above-described autorefraction measurement, while others theorize that the light passes through the choroid and scatters off of the sclera. Regardless of the exact location at which the light scatters (hereinafter referred to as “the scattering location”), there is a definite distance between the scattering location and the photoreceptors. This phenomenon may be understood with reference to FIG. 5, which shows an infrared probe beam 25 passing into the eye 500 through the cornea 540 and lens 530, passing through the fovea 520, and striking the choroid 510. In FIG. 5, it is shown that the light scatters off the choroid 510 instead of the photoreceptors, although as explained above, the principle applies regardless of the actual scattering location.
Second, the eye works at visible wavelengths, but the infrared radiation is subject to chromatic aberration and that changes the refraction values. Fortunately, many papers have been published on the effects of chromatic aberration in the human eye. It is relatively easy to use the published data to make accurate adjustments to the refraction calculation based on published chromatic aberration values.
It is hypothesized that the significant structural difference in the 20% of patients with significant differences between subjective refraction and autorefraction is that the spacing between the photoreceptors to the scattering location is different in those patients relative to the general population. However the literature on the spacing or distance between the photoreceptors and the scattering location is much more sparse, and does not extend much past a few brief paragraphs in journals and textbooks. The effect of the chromatic aberration is such that the raw measurement will measure incorrectly by about negative 1.4 Diopters. The effect of the spacing or distance between the photoreceptors and the scattering location is such that the raw autorefractor value will be in error by about a positive 0.8 Diopters. The combined effect of the two adjustments is that the raw measurement from the autorefractor needs to be adjusted by about negative 0.6 diopters in order to agree with the subjective refraction.
Meanwhile, the Stiles-Crawford effect is also suggested as being responsible for causing discrepancies between subjective refractions and autorefractions. The Stiles-Crawford effect refers to the fact the cones in the eye show a marked preference to respond to light that is within a relatively narrow range of angles. (The fovea is where high resolution vision occurs and it is packed very densely with cones.) The effect is such that a ray of light entering the edge of a 7 mm pupil will cause a response that is about 22% as strong as a ray that enters the center of the pupil.
In normal eyes, the photoreceptors are pointed so that the peak response is pointed to somewhere in the center 1.0 mm region of the pupil. However, it can happen that the cones point toward the edge of the pupil.
One hypothesis is that the autorefractors measure inaccurately because they calculate the sphere cylinder and axis paying special attention to weight the light in the center of the pupil the most strongly. But consider the possibility that the eye is really weighting the lower half of the pupil more heavily than the center. If the sphere value in that region of the eye is one diopter different than it is in the center of the pupil, the autorefractor would read incorrectly by one diopter. Wavefront aberrometry measurements on patients can indicate how much of a difference might be caused in a subjective refraction due to a Stiles-Crawford effect.
Objective and subjective methods have been developed to evaluate the strength of the Stiles-Crawford effect, and to locate the position on the eye's pupil that is weighted the most strongly in vision. The knowledge of that location, along with a refractive power map derived from a wavefront aberration map, can be used to calculate improved values of sphere cylinder and axis that would better correspond to those that would be obtained by a subjective refraction.
One would expect that only subjects with large high order aberrations would be affected by the Stiles-Crawford effect. However, a number of subjects have been measured that have significant differences between subjective and objective refractions and they had very small high order aberrations. This observation supports the view that variations in the distance between the scattering layer and the photoreceptors is a primary reason for differences in objective and subjective refractions in those subjects, although the Stiles-Crawford effect still may play a role in some subjects.
Another factor that affects the accuracy of an autorefraction is the distance from the eye to the instrument. Autorefractors typically contain a method to assist the doctor in setting this distance to the optimal value. Many other instruments such as corneal topographers contain very accurate methods of setting that distance. One of the simplest being a camera that looks at the head from the side so that the cornea is seen in profile and the instrument moved back and forth until the apex of the cornea lines up with a reticle on a video screen.
Accordingly, it would be desirable to provide a method and system to measure the spacing or distance between the photoreceptors and the scattering location during an objective refraction measurement, and a method to use that parameter to improve the calculated spherical equivalent power on those patients. It would also be desirable to provide an instrument including an optical beampath for testing the hypothesis that the Stiles Crawford effect is responsible for the difference in autorefractions versus subjective refractions.
The present invention comprises a system and method for measuring a distance between the photoreceptors and the scattering location in an eye. Beneficially an objective refractor is employed to perform an objective refraction of the eye and to measure the distance between the photoreceptors and the scattering location in an eye. The objective refractor could be an autorefractor, a wavefront aberrometer, a photoretinoscope, or a similar device that relies on objectively measuring the eye.
In another aspect of the invention, a method for measuring a distance between the photoreceptors and the scattering location in an eye comprises performing an autorefraction of the eye with an objective refractor; focusing the eye on a rotating speckled light pattern; adjusting a distance between the speckled light pattern and the eye until the speckled light pattern appears to be stationary; measuring the distance between the speckled light pattern and the eye when the speckled light pattern appears to be stationary; and calculating the distance between the photoreceptors and the scattering location based on the distance between the speckled light pattern and the eye.